Method of studying chemical reactions by measuring interfacial film thicknesses



. H. J. TRURNIT 66 METHOD OF STUDYING CHEMICAL REACTIONS BY MEASURINGINTERFACIAL FILM THICKNESSES 2 Sheets-Sheet 1 INVENTOR. Hans J. fi'um/fJan. 19,1954

Filed Oct. 12, 1951 Jan. 19,- 1954 H. J. TRURNlT 2,666 3 METHOD OFSTUDYING CHEMICAL REACTIONS BY MEASURING INTERFACIAL FILM THICKNESSESFiled Oct. 12, 1951 2 Sheets-Sheet 2 zzvmvroa Hans J. 770ml! ATTORNEYPatented Jan. 19, 1954 PATENT OFFICE- 7 666,355 MET'HODA' om s'i tmyl-NdCHEMICAL REAC- IIONStU-BYQ- MEASURING INTERFAGI-AL FILM THICKNESSESApplidtiondci kfihgifi,195E'SeriafNi'ZShR 5 61x555 (Gi i-" 1 1 (ei-anfaaufidr Title 35, U. s. 0661: (1952');

see. 266

a? method for si u dyi we n Thus if ordinary ight 'is" l flectedf 1'1"e, a' n interfere hm Whi'ch'v'ar ship is true when the angle of iaecasss employing monochromatic light as the beam the wave lengththereof will be constant. Further, by passing the reflectedplane-polarized light through an analyzer and rotating the same withreference to the compensator until the light passing through reaches aminimum intensity, the azimuth and thus the film thickness may bedetermined. An apparatus for doing this is termed an ellipsorneter andis the basis of the Rothen method.

The use of the foregoing methods is successful in determining thechanges in film thickness of various reactants. Thus it was possible bymeasuring the thickness of an absorbing film before it was placed in areacting field and again after it was removed from the field todetermine the amount of material absorbed thereon. This method. haslimitations however in that the stood however that the description isfor illustration purposes and I do not wish to limit the scope of myinvention thereto.

In the drawing which employs a polarizer and a metallic reflecting plateand therefore reflects elliptically polarized light, I is glass lenspositiohed between the light source and entrance opening of themonochromator 2 so that it will measurements are periodic and notcontinuous. f

I have found that by passing the light through the reaction situs, i.e., the reaction interface, while the reaction is taking place,positioning a highly sensitive photo cell in the path of the reflectedlight to transpose the intensity into voltage potential and empiricallyrelating the film thickness to a calibrated standard material under thesame conditions, it is possible to continuously record the reaction andintimately study the rate thereof. Thus it is no longer necessary toremove the reflecting slide from the reaction solution to measure thechange in film thickness. The photo cell is sensitive enough to measurechanges in film thicknesses of the order of a few angstrom units and byemploying an empirical calibration technique, the factors in- 1,;

troduced by additionally passing the light through the reaction mediaare canceled as they are independent of the film material and thethickness thereof. i

In employing this technique, it is important to observe that there aresome limitations imposed thereon. The technique is successful ifemployed with a small number of superimposed monomolecular films onlyand the reaction media must be transparent, such as water for a liquidor air for a gas. Of course, the film to be studied must likewise betransparent, but no difficulty is encountered here, as all films are inthe neighborhood of molecular thickness and are void of colloidalparticles, the chief cause of opaqueness. In addition, the calibratingmaterial, which also serves as a base material on which the film isdeposited, must be transparent. I have employed barium stearate as mycalibrating and base material, but it is to be understood that my methodis not limited thereto. Many other organic and inorganic materials couldbe successfully employed such as metallic stearates, oleates,myristates, the calcium salts for example are satisfactory. Anylong-chained fatty acid molecules which form rigid oriented monolayerscan be successfully employed.

Referring now to the drawing, Fig. 1 is a hori zontal sectional view ofthe apparatus employed in recording the reaction kinetics taken alongthe line |-i of Fig. 2.

Fig. 2 is a vertical sectional view taken along the line 2-2 of Fig. 1.

Fig. 3 is a graphic representation of the results obtained in Example I.V

Fig. 4-. is a graphic representation of the results obtained in ExampleII.

In describing my invention I have employed the principle of ellipticalpolarization and measured the parameter alteration. It is to beundergoing parallel light convergent.

focus the light at the said opening. The monochromator consistsessentially of a rectangularly shaped box or housingv having tubularshafts 3 and 4 positioned on perpendicularly opposed sides of thebox andin a common plane. The shafts are open at their ends. A prism 5 islocated within the box at the intersection point of axial projectionsfrom the shafts. In each of the shafts there is located a lens forrendering the diverging incoming light parallel and out- The prism islocated on a rotatable bench and be revolving the bench, any particularlight band may be selectively emitted at the end of shaft 4. Themonochromatic light enters a third shaft 6 which is slitted. At the endof shaft 5, a polarizer l is located across the path of the beam travelwhich may be made of double refractory crystalline material, such asNicol, etc. Rearward thereof is located a lens for rendering thedivergent beam parallel. Located beyond the open end of shaft 5 is arotatable and vertically movable table 8 with suitable locking meanstherefor and upon which a cuvette or reaction glass is placed. Thereflecting plate is placed in the cuvette in such a manner that thelight leaving shaft 6 passes through the fluid, is reflected off thefilm and metal surface and enters a fourth shaft 9. Shaft 9 is similarto shaft 6 being open at the end adjacent to table 8 and slitted at theopposite end. In the shaft 9 adjacent the open end thereof andintercepting the light passing therethrough, there is located acompensator it which converts the elliptically polarized light toplane-polarized light. The compensator is a thin disc of a doublerefracting crystal plate of crucial thickness protected by two glassdiscs; this unit is commonly known as the quarter-wave length plate.Beyond the plate and parallel thereto there is located an analyzer Hwhich is nothing more than a disc of polarizing material. By rotatingdiscs ii) and II until no light passes through the analyzer andemploying this as a base whereby any change in thickness of film willproduce light of increasing intensity, it is possible to get the maximumsensitivity possible as the calibrating curve, which is in the form of asine wave is beginning at this point to move upward and. each horizontalvariation will record a maximum vertical change. It being understood ofcourse that in so far as the invention is concerned that presetting thesystem for maximum sensitivity is not necessary. Beyond the analyzerthere is located another lens to render the rays divergent. Thecombination of the shafts 6 and 9 with the associated polarizers andcompensator together with the reflecting surface is popularly referredto as the ellipsorneter. Located adjacent to the slitted end of shaft 9is a photo-electric cell if of high sensitivity such as photo-multipliercell. This cell isused to convert light intensity into electricalpotential. The cell is attached to an amplifier i3 which in turn isattached to a recorder M for the well-known purpose of boosting thepotential generated in the photo-electric cell whereby it will actuatethe recorder.

a ee e sre'cordersmays ny-rztype .=.of:. =highespeed egalganometcrshaving a ,5 circular .5101 1, rectilinear."chartiattachedrtheretoilortzany eother :meansfforcvisualcobseizvance.iofrvoltageg fluctuations.

In ;.=actual':.:practice, lightwhters ilens l, $15 Iefocusedliref-ractediandzthesbandmeshed is prelsented'aat :the entrance.;:of the c: ellipsometer, susua'lly :::the' pyellow t-sodium band.fist-employed it :wculdinterf ere swith-ir the-ecoloruof :itheixeaction: fluid. JPIn.etherellipsometer,itheiilight'zis--plane-polarized,- :reflectedias elliptically:polarizedtlighwtransposed into plane:-polarizedjight convertediint'o .electrical:potential: and; the :intensity rthereof recorded.

' 'i hesfollowing examples flwill zdemonstrate z the ite'chniqueseemployed and illustratei.the ezlfective- -ness ofi thesystem.

Example =I base layer was placed 'a six molecule layer of bovine serumalbumin as theantigenic protein, =thus maintaining the-50step'increments.

The plate wasilongitudinallycplaced into the ellipspmeter cuvette which-cntained distilled wat 'l andthus by moving the cuvette up or.downrelative to the light beam, the selected step could be placed in the:path of the beam. The vertical moving was accomplished by having theellipsometer turntable mounted on a low pitch screw.

After placing the cuvette on the turntable, the recording paper wascalibrated by exposing the photocell to the reflected light of one step(step 2). After zero stabilization of the photo-volt amplifier, threemore steps were consecutively exposed to the reflected light and theprocedure was reversed. Since the steps were 50 A. the vertical steps ona film thickness-time recording chart would represent 50 A.calibrations. The chart was set to move at a convenient rate so that aclear picture could be obtained. The chart shown in Fig. 3 representsthe calibration results of this sample; the abscissa indicates time inhalf minutes and the ordinate film thickness in A. Thus in section A ofthe recording chart steps 2, 3, 4, and 5 were employed to calibrate thecell with a step wise return to step 2 on which the experiment tookplace. It will be noted that the starting point of the experiment isrecorded above the calibrating position for the same step, this is dueto adjusting the photo-volt amplifier to provide space below thestarting level if needed; it does not affect the result.

With the cuvette in position on the turntable, and the stirrer set inmotion, sodium chloride solution is added to bring the totalconcentration in the cuvette to 0.9%. The spike at B (Fig. 3) is due tothe sudden inhomogeneity of the refractive index of the solution; ittakes several seconds before the stirrer is able to homogenize thesolution. The ewalevelsafterg-B corresponds etorthenewfrefractiverindex.

' "After ..several :minutesgOzl .;cc. ofea high; titer specific. rabbit.ianti'ebovine. serum albumin-serum .was added tocthe cuvette whichyields 1 an :-;i=m ymediatezsmall adsorption. nThis 1 small aincreasecafterrstirri-ng. gives a new :level on; theichart as';seen..;at;;C.-;due. to theslight increase in fllm:;thick-..-.minute'ilater-ra small amount ofphoslphatezchuifer was .iadded zto.1 the .sol-ution .which :rernoyed :pa-rtof the. surface: material;resultingg-in raidropebelow; the :readinglevel of theechart at D.

The zero was again readjusted .and aftenseveral .-.-minutes, ;:.moreanti-serum was added hand ithe zreactionc rate-was recordedifor over aquarter of canchjour. :eThisisshown to the right of 'Esont zthe---figure.

.r Example -II n.il .s...ei amp1e. .ar e d n wasmad poflth :kin'e qs voredzin t e 'h d e ysis of, e ac ;.s nj crr tein y. the enzym try .25. pat was pr pared accord ng .tof-t metho .se jforth inv Ex mpl hI ab e empov n b rin serum albumin asthe protein substrate.

jllhe photo cell was exposed-to the reflected light ofv two adjacentsteps twice. This is. indicated;atA on Fig. llwherein .the A. increments.ar mue larger t a 6 715, 3- .ln; addit en th time divisions epresen:Qneminute athe aa alfim nutelalsa ne ed'thete s Qs e ,-.th aphet l ddrn l c uc th sam c on 0 e. s v ;ah '.-t1;. .e,re ".WfiSr .vlsherber ine o the ;;seeend; ria1- Thi wa e ach need by: diust c t ephoto vol p1ifier.

A S e am un ph phetebuf er was-adde to. the cuvette toremfovesome of.theprotein this is indicated at C on Eig. .4,.where, afterhornggenization, the curve went slowly downward; this was due to thedecrease in film thickness and change in the refractive index of thesolution.

After six minutes the conditions had become constant. At this time, twomilligrams of dry trypsin were added to the cuvette and within oneminute the protein was hydrolized. This change was indicated on thechart at D where there was a sudden drop in the curve followed by animmediate and continuous stabilization.

From the foregoing description and examples, it will be seen that I havedeveloped a method and apparatus combination to continuously record andstudy the kinetics of chemical surface reactions.

Having described my invention, I claim:

1. A method of continuously measuring the thickness of a transparentfilm which is subjected to a reactant in a reaction media whichcomprises building a step series of known molecular size increments ofthe film upon a reflecting surface, placing said surface in atransparent reaction media, separately exposing different steps to abeam of plane-polarized monochromatic light transparent base materialupon a reflecting surface, superimposing a molecular size layer of thefilm upon the base material, placing the reflecting surface in atransparent reaction media, separately exposing different steps of thebuilt up film to a beam of plane-polarized monochromatic light, therebyproducing reflected beams of light of difierent intensity, exposing alight intensity recorder to the reflected light beams, therebycalibrating the said recorder, exposing one of the step layers of thefilm to the polarized light, adding the co-reactant to the reactionmedia and recording the intensity of the reflected light.

3. A method of continuously measuring the thickness of a transparentfilm which is subjected to a reactant in a reaction media whichcomprises building a step series of known molecular size increments ofthe film upon a metallic reflecting surface, placing said surface in atransparent reaction media, separately exposing different steps to abeam of plane-polarized monochromatic light thereby producing reflectedbeams of elliptically polarized light, converting the ellipticallypolarized light to a plane-polarized light of different intensities,exposing a light intensity recorder to the reflected light beams therebycalibrating the said recorder, exposing one of the step layers of thefilm to the polarized light, adding the coreactant to the reaction mediaand recording the intensity of the reflected light.

4. A method of continuously measuring the thickness of a transparentfilm which is subjected to a reactant in a reaction media whichcomprises building a step series of known molecular size increments ofthe film upon a non-metallic reflecting surface, placing said surface ina transparent reaction media, separately exposing difierent steps to abeam of plane-polarized monochromatic light thereby producing reflectedbeams of plane-polarized light of different intensity, exposing a light8 intensity recorder to the reflected light beams thereby calibratingthe said recorder, exposing one of the step layers of the film to thepolarized light, adding theco-reactant to the reaction media andrecording the intensity of the reflected light.

5. A method of continuously measuring the thickness of a transparentfilm which is subjected to a reactant in a reaction media whichcomprises building a step series of known molecular size increments ofthe film upon a metallic reflecting surface, placing said surface in atransparent reaction media, separately exposing different steps to abeam of plane-polarized monochromatic light thereby producing reflectedbeams of elliptically polarized light, passing the ellipticallypolarized light througha compensator thereby converting the ellipticallypolarized light to plane-polarized light of different intensities,passing the planepolarized light through an analyzer which is orientedso that its azimuth is intermediate between the azimuth of the major andminor axes of the compensator whereby the difierent intensities of theplane-polarized light are more sharply defined, exposing a lightintensity recorder to the reflected light beams thereby calibrating thesaid recorder, exposing one of the step layers of the, film to thepolarized light, adding the co-reactant to the reaction media andrecording the intensity of the reflected light.

722,746 Germany -l July 20, 1942

